# std::match_results

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< cpp‎ | regex

Defined in header `<regex>` |  |
template< class BidirIt, class Alloc = std::allocator<std::sub_match<BidirIt>> > class match_results; | (1) | (since C++11)
namespace pmr { template <class BidirIt> using match_results = std::match_results<BidirIt, std::pmr::polymorphic_allocator< std::sub_match<BidirIt>>>; } | (2) | (since C++17)

The class template `std::match_results` holds a collection of character sequences that represent the result of a regular expression match.

This is a specialized allocator-aware container. It can only be default created, obtained from std::regex_iterator, or modified by std::regex_search or std::regex_match. Because `std::match_results` holds std::sub_matches, each of which is a pair of iterators into the original character sequence that was matched, it's undefined behavior to examine `std::match_results` if the original character sequence was destroyed or iterators to it were invalidated for other reasons.

The first std::sub_match (index 0) contained in a `std::match_result` always represents the full match within a target sequence made by a regex, and subsequent std::sub_matches represent sub-expression matches corresponding in sequence to the left parenthesis delimiting the sub-expression in the regex.

`std::match_results` meets the requirements of a AllocatorAwareContainer and of a SequenceContainer, except that only copy assignment, move assignment, and operations defined for a constant containers are supported, and that the semantics of comparison functions are different from those required for a container.

### Type requirements

- `BidirIt` must meet the requirements of LegacyBidirectionalIterator.
- `Alloc` must meet the requirements of Allocator.

### Specializations

Several specializations for common character sequence types are provided:

Defined in header `<regex>`
Type | Definition
`std::cmatch` | std::match_results<const char*>
`std::wcmatch` | std::match_results<const wchar_t*>
`std::smatch` | std::match_results<std::string::const_iterator>
`std::wsmatch` | std::match_results<std::wstring::const_iterator>
`std::pmr::cmatch` (C++17) | std::pmr::match_results<const char*>
`std::pmr::wcmatch` (C++17) | std::pmr::match_results<const wchar_t*>
`std::pmr::smatch` (C++17) | std::pmr::match_results<std::string::const_iterator>
`std::pmr::wsmatch` (C++17) | std::pmr::match_results<std::wstring::const_iterator>

### Member types

Member type | Definition
`allocator_type` | `Allocator`
`value_type` | std::sub_match<BidirIt>
`const_reference` | const value_type&
`reference` | `value_type&`
`const_iterator` | implementation-defined (depends on the underlying container)
`iterator` | `const_iterator`
`difference_type` | std::iterator_traits<BidirIt>::difference_type
`size_type` | std::allocator_traits<Alloc>::size_type
`char_type` | std::iterator_traits<BidirIt>::value_type
`string_type` | std::basic_string<char_type>

### Member functions

(constructor) | constructs the object (public member function)
(destructor) | destructs the object (public member function)
operator= | assigns the contents (public member function)
get_allocator | returns the associated allocator (public member function)
##### State
ready | checks if the results are available (public member function)
##### Size
empty | checks whether the match was successful (public member function)
size | returns the number of matches in a fully-established result state (public member function)
max_size | returns the maximum possible number of sub-matches (public member function)
##### Element access
length | returns the length of the particular sub-match (public member function)
position | returns the position of the first character of the particular sub-match (public member function)
str | returns the sequence of characters for the particular sub-match (public member function)
operator[] | returns specified sub-match (public member function)
prefix | returns sub-sequence between the beginning of the target sequence and the beginning of the full match (public member function)
suffix | returns sub-sequence between the end of the full match and the end of the target sequence (public member function)
##### Iterators
begincbegin | returns iterator to the beginning of the list of sub-matches (public member function)
endcend | returns iterator to the end of the list of sub-matches (public member function)
##### Format
format | formats match results for output (public member function)
##### Modifiers
swap | swaps the contents (public member function)

### Non-member functions

operator==operator!= (removed in C++20) | lexicographically compares the values in the two match result (function template)
std::swap(std::match_results) (C++11) | specializes the `std::swap` algorithm (function template)

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